--- a/parser/html/nsHtml5StreamParser.cpp
+++ b/parser/html/nsHtml5StreamParser.cpp
@@ -957,17 +957,17 @@ nsHtml5StreamParser::OnStartRequest(nsIR
   // The line below means that the encoding can end up being wrong if
   // a view-source URL is loaded without having the encoding hint from a
   // previous normal load in the history.
   mReparseForbidden = !(mMode == NORMAL || mMode == PLAIN_TEXT);
 
   nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(mRequest, &rv));
   if (NS_SUCCEEDED(rv)) {
     nsAutoCString method;
-    httpChannel->GetRequestMethod(method);
+    Unused << httpChannel->GetRequestMethod(method);
     // XXX does Necko have a way to renavigate POST, etc. without hitting
     // the network?
     if (!method.EqualsLiteral("GET")) {
       // This is the old Gecko behavior but the HTML5 spec disagrees.
       // Don't reparse on POST.
       mReparseForbidden = true;
       mFeedChardet = false; // can't restart anyway
     }